1. Field of the Invention
This invention relates to a compact extruder with multiple parallel screws having a variable pitch arrangement.
2. Description of the Related Art
At present, extruders of various types, especially twin-screw extruders, are used in the extrusion technique employed for materials in general.
These types of known extruders, which are used, for example, for materials which are not easily mixed and contain volatile substances, are somewhat large; their overall dimensions can be up to 30-45 times the diameter of the screw.
In order to make these extruders shorter, specially shaped screws with variable cross-sections have been designed; however, the manufacture of such screws is rather complex.
The costs of manufacturing and assembling such extruders are equally high in view of the length of the extruders, the component parts and the associated assembly, running and maintenance costs.
The purpose of this invention is to make an extruder with two or more screws which simply and rapidly solves the problems briefly described above in general.
Another purpose of this invention is to make an extruder with small overall dimensions and short length which is compact and can be adapted for various treatments of different materials.
These and other purposes are achieved by this invention, which relates to a compact extruder with multiple parallel screws, comprising an outer casing which includes at least two chambers; the chambers contain at least two screws, which allow material to flow from one chamber to the other through a passage, and present a feed inlet for the material to be treated by the extruder and an outlet for the material; the extruder is characterized in that each chamber is constituted by at least one pair of housings for the corresponding pair of screws which engage with one another, each screw presenting a long-pitch portion and a short-pitch portion.
The long-pitch portion preferably has a constant pitch. Advantageously, the long-pitch portion will also be longer than the short-pitch portion. In particular, the short-pitch portion is of variable pitch.
In a special, preferred embodiment of the invention, the variable short-pitch portion has sections with opposite threads.